Skip to main content

Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

See other formats

washed up over a bank 5 to 25 feet in height and rushed back 100 to 200 feet into the alder thicket, which is much farther than the waves reached in 1909. Much gravel and sand, stones a foot or two in diameter, and ice blocks (PI. CL, B) up to ten tons in weight were thrown in among the trees. Alders, 9 to 11 inches in diameter, were stripped of leaves and bark, and bent backward or broken off short, or uprooted, or buried beneath the gravel and bowlders. The river bank, which at some points of measurement was cut back from  to 5 feet in 1909, was fairly eaten up by the iceberg waves which crossed the river in 1910, as is proved by measurements at the same points along the bank of the stream facing the glacier, where the recession of the river bank was from 40 to 60 feet.
The amount of retreat of the river bank is shown in the following tables, the 1909 measurements having been made by the railway engineers, while those of 1910 were made by our topographer from points on the same pair of short baselines which Mr. Johnson established the year before. The material of the river bank is very coarse, unconsoli-dated, glacial gravel and bowlders. The points of observation are on the eastern bank of Copper Biver, nearly opposite the middle of Childs Glacier.
Place of Measurement	July 19, 1909	Aug. 1, 1909	Oct. S3, 1909	Aug. 18, 1910
Pt. A to river bank	44 feet	41 feet		In river
Pt. B    "     "	63   "	58   "		In river
Pt. E   "     "	67   "	66   "	65ifeet	16 feet
Pt.F    "     "	64    "	49   "	45*    "	-15 feet
Pt. E is the only one of the points of measurements that was left in 1910, the river bank having receded beyond the sites of the other three, though in the case of Pt. F the position was determined exactly by measurement of direction and distance from Pt. E. These measurements show that the increase in iceberg discharge in the river, which was narrowed by the advance of the glacier, resulted in retreat of the river bank as follows:
1909 AND 1910
Days Between Observations	Pt. A	Pt. B	Pt.E	Pt. F
July 19 to Aug. 1, 1909, 13 days	3 feet	5 feet	If oot	5 feet
Aug. 1 to Oct. 23,           83 days			$ foot	sift.
Oct. 23 to ------ ,* 1909,				
------ 1 to Aug. 18, 1910      days	Over 41 ft.	Over 58 ft.	49 ft.	60$ ft.
In addition to the retreat at these points there was in 1910 a general retreat of the whole river bank facing the glacier, shown particularly well by the undercutting and
i Dates between which the river was closed by ice, so that wave work ceased, though there may have been some erosion of the river bank by the ice blocks which were shoved against the river during the winter.